The present invention relates generally to amplifiers, and more specifically to amplifiers with programmable gain.
Amplifiers are commonly used to produce an output voltage or an output current in response to an input voltage or an input current. Voltage amplifiers receive an input voltage and produce an output voltage. Current amplifiers receive an input current and produce an output current. Other types of amplifiers also exist. For example, a transconductance amplifier receives an input voltage and produces an output current.
FIG. 1 shows a prior art amplifier circuit. Amplifier circuit 100 includes operational amplifier (op-amp) 102, feedback resistor (Rf) 104, and input resistors (R1, R2) 106 and 108. Amplifier circuit 100 produces an output voltage (VOUT) on node 110 from input voltages (VIN1, VIN2) on nodes 112 and 114. The output voltage satisfies the following equation:                               V          OUT                =                                            (                                                -                                      R                    f                                                                    R                  1                                            )                        ⁢                          V              IN1                                +                                    (                                                -                                      R                    f                                                                    R                  2                                            )                        ⁢                          V              IN2                                                          (        1        )            
As shown in equation (1) above, amplifier circuit 100 scales (or xe2x80x9cmultipliesxe2x80x9d) each input voltage by a constant value and sums the scaled voltage values. The constant values used to scale the input voltages are equal to a ratio of resistance values. By varying the resistance values of resistors 104, 106, and 108, the input voltage scaling can be changed.
As is known in the art, amplifier circuit 100 has many uses. It is also known in the art that amplifier circuit cannot operate at extremely high frequencies, in part because op-amp 102 usually includes compensation circuits to avoid instability, and these compensation circuits tend to limit the frequency at which the op-amp can operate.
Other example circuits that provide voltage multiplication include the xe2x80x9cGilbert cellxe2x80x9d as described in chapter eight of: David A Johns and Ken Martin, xe2x80x9cAnalog Integrated Circuit Design,xe2x80x9d (1997).
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for methods and apparatus to provide amplifiers and multipliers that operate at high frequencies.